The invention relates to a hydraulic actuating device for a movable component of a vehicle, in particular a closure element for closing an opening in a vehicle body, such as for example a movable roof, such as a folding roof or a retractable hard top.
The applicant is aware of a hydraulic actuating device as shown in FIGS. 1a and 1b. 
FIG. 1a shows a hydraulic actuating device comprises an actuator 1, with the first working chamber 6 being in communication, via a first connection 4 and a first connecting line 15, with pressure port 11 of pump 10. A second working chamber 7 is connected to a second connecting line 16 via a second connection 5. In this connecting line 16 there is a control valve assembly 19. This control valve assembly 19 ensures that the second working chamber 7 can be connected either to the pressure port 11 of the pump 10 or to a reservoir 12. The control valve assembly 19 is driven by a control device 50.
If, in the known hydraulic actuating device shown in FIG. 1a, it is desired to retract piston rod 3 of the hydraulic actuator 1, the control device 50 switches the control valve assembly 19 in such a manner that the second connecting line 16 connects the second working chamber 7 of the actuator 1 to the reservoir 12. However, if it is desired to extend the piston rod 3, the control device 50 has to switch the control valve assembly 19 in such a manner that the connecting line 16 connects the second working chamber 7 to the pressure port 11 of the pump 10. It can be seen from FIG. 1a that the pressure which is applied by the pump 10 in that situation acts on both sides of piston 2 of actuator 1. On account of the difference in active surface area of the two sides of the piston 2, which difference is caused by the presence of piston rod 3, a low resulting force remains, allowing the piston to move in such a manner that the piston rod 3 is extended.
One significant drawback of this hydraulic actuating device is that the resulting force with which the piston rod is extended is low, since during the extension the pressure acting on the piston on the piston rod side counteracts the pressure acting on the piston from the base side. The effective surface area of the piston on which the hydraulic pressure acts is therefore limited, when the piston rod is being extended, to a surface area equal to that of the cross section of the piston rod.
To overcome this drawback, it has in the past been proposed that a control valve assembly 18 identical to the control valve assembly which is present in the second connecting line also be incorporated in the first connecting line as is shown in FIG. 1b. This control valve assembly 18 is driven by control device 50. However, this makes the actuating device more expensive and more complex to drive. A hydraulic actuating device of this type is known, for example, from EP 1052127.